The priming of the TH1, rather than TH2, subset of CD4+ T cells is of primary importance in vaccine design. This is because TH1 cells produce IL-2, IFN-γ, and TNF-β cytokines that mediate macrophage and cytotoxic T cell activation (CTL), and are the principal effectors of cell-mediated immunity against intracellular microbes and of delayed type hypersensitivity (DTH) reactions. IFN-γ also induces B cell isotype class-switching to the principal effector isotype of mouse IgG. IgG2a, and to a lesser extent IgG2b, enhances antibody-dependent cell mediated cytotoxicity (ADCC) and strongly binds Clq of the classical complement pathway which opsonizes cells or antibody clusters for phagocytosis. For example, as a result of these effector functions in mouse, IgG2a has been found to better protect mice against virus infections (Ishizaka et al. (1995) J. Infect. Dis. 172:1108), murine tumors (Kaminski et al. (1986) J. Immunol. 136:1123), and parasites (Wechsler et al. (1986) J. Immunol. 137:2968), and to enhance bacterial clearance (Zigterman et al. (1989) Infect. Immun. 57:2712).
In contrast, TH2 cells produce IL-4, IL-5, IL-10, and IL-13, which have the undesirable effect of suppressing cell mediated immunity (Mossman et al. (1986) J. Immunol. 136:2248). Furthermore, IL-4 induces B cells to produce both an IgG subclass that poorly fixes complement and does not mediate ADCC, as well as IgE that binds to mast cells and basophils.
The prior art has attempted to improve vaccine design by directing the development of TH1 cells, while recognizing that the priming of TH cell subsets is affected by the strain of animal used (Hocart et al. (1989) J. Gen. Virol. 70:2439), the identity of the antigen, the route of antigen delivery (Hocart et al. (1989) J. Gen. Virol. 70:809), and the immunization regimen (Brett et al. (1993) Immunology 80:306).
One approach of the prior art to generate antigen-specific TH1 responses has been through the use of the oxidative/reductive conjugation of mannan to antigen (Apostopoulos et al. (1995) Proc. Natl. Acad. Sci. USA 92:10128-10132) or the conjugation of proteins to bacterial proteins (Jahn-Schmid et al. (1997) Intl. Immunol. 9:1867). However, coupling to commonly used carriers such as KLH and tetanus toxoid, is often unsuccessful at increasing the IgG2a:IgG1 ratio.
Other methods of inducing TH1 responses include the use of immunomodulatory agents, such as extraneous adjuvants (for example: ISCOMS, QS-21, Quil A, etc.). However, alum is the only adjuvant which is currently approved for use in humans and is known to favor undesirable TH2-type responses rather than the more desirable TH1-type response.
Other immunomodulatory agents that have been used by the prior art include CpG oligodeoxyribonucleotides (Chu et al. (1997) J. Exp. Med. 186:1623) or cytokines, such as interleukin-12 (IL-12, Scott et al (1997) Semin. Immunol. 9:285), which may be added to immunogenic compositions leading to the production of high levels of IgG2a directed against soluble protein antigens. However, the cytokines' short half-life and considerable cost make utilizing them both technically and commercially unattractive in large-scale vaccination.
Yet another approach has been to use live animal virus vaccines to generate predominantly virus-specific IgG2a in mice (Hocart et al (1989) J. Gen. Virol. 70:809; Coutelier et al. (1987) J. Exp. Med. 165:64; Nguyen et al. (1994) J. Immunol. 152:478; Brubaker et al. (1996) J. Immunol. 157:1598). This approach has several disadvantages. First, live virus vaccines do not consistently result in a TH1-type immune response, since some live viruses favor production of other immunoglobulin isotypes characteristic of alternate T helper pathways (Coutelier et al. (1987) J. Exp. Med. 165:64; Perez-Filgueira et al. (1995) Vaccine 13:953). In addition, such animal virus vaccines are produced from viruses that are grown in cell culture systems that are expensive to design and run.
Moreover, the animal virus used as the vector is often a virus to which the animal may already have been exposed, and the animal may already be producing antibodies to the vector. The vector may, therefore, be destroyed by the immune system before the incorporated antigenic site of the second virus induces an immune response. Additionally, the composite animal virus approach involves genetic manipulation of live, animal-infecting viruses, with the risk that mutations may give rise to novel forms of the virus with altered infectivity, antigenicity and/or pathogenicity. Indeed, there are safety concerns over the use of live animal viral vaccines (World Health Organization, 1989). Furthermore, the safety concerns over the use of live animal viral vaccines are not overcome by using inactive animal viruses since inactive animal viruses generally do not favor IgG2a production. Indeed, it is thought that the infection process typified by live viruses per se generates IFN-γ leading to a predominance of TH1 response immunoglobulins (Nguyen et al. (1994) J. Immunol. 152:478).
While another approach has involved using live bacterial vectors and DNA immunization that favor the generation of TH1 responses to expressed peptides, this approach is, however, often dependent on, and sensitive to, either the route of delivery or adjuvant used. Moreover, safety concerns over the use of live bacterial vaccines and DNA vaccines (World Health Organization, 1989) further limit their clinical application.
Thus, there is a need for compositions and methods of generating TH1-type responses. Preferably, the generation of TH1-type responses by these compositions and methods is unaffected by the genetic background of the host animal, the identity of the antigen, the route of antigen delivery, and the immunization regimen.